1. Field of the Invention
The present invention relates generally to brush holders for vehicular a.c. generators and more particularly to a brush holder having a brush box for holding brushes therein and compression springs for respectively urging the brushes.
2. Description of the Prior Art
FIG. 5 is a sectional view of a vehicular a.c. generator fitted with a brush holder. As shown in FIG. 5, the a.c. generator comprises a stator 1 formed of a stator core 2 and a stator coil 3, a rotor 4 formed of heteropolar magnetic cores 5, 6, the magnetic tooth portions of both being alternately projected in the circumferential direction, an exciting coil 7 held between the magnetic coils 5, 6, a rotary shaft 8 for securing the magnetic cores 5, 6, a pair of slip rings 9 held by the rotary shaft 8, and fans 10, 11 respectively held by the magnetic cores 5, 6.
The generator further includes a front and a rear bracket 12, 13 for fixing the stator core 2 by means of a clamping bolt 14, bearings 15, 16 for supporting the rotary shaft 8 respectively on the front and the rear bracket 12, 13, a pulley 17 fixed to the rotary shaft 8 and used for transmitting torque to the rotary shaft 8 in order to rotate the rotor 4.
The generator further includes a rectifier 18 for rectifying a.c. current resulting from the a.c. voltage induced in the stator coil 3.
The generator further includes a voltage regulator 19 for detecting the generator voltage, controlling exciting current, and adjusting terminal voltage to a predetermined value.
The generator further includes a brush holder 20 fixed to the rear bracket 13 and used for longitudinally movably urging and holding brushes 28, and a partition ring 22 fixed to the rear bracket 13 in such a way as to screen the generator from injurious materials such as atmospheric moisture having a salt content, and the like, by covering the outer circumference of the slip rings 9, with a space provided therebetween.
FIG. 6(a) and 6(b) are a partial cutaway elevational view and a side sectional view of a conventional brush holder fitted to the vehicular a.c. generator described above. In this case, the brush holder is in such a state that it has been removed from the a.c. generator. The brush holder 20 comprises a brush box 21 made of synthetic resin, the brushes 28 being inserted therein, a connector unit 23, a terminal unit 24, compression springs 25 for urging the respective brushes 28, a cover 27 made of synthetic resin, and connecting leads 29 connected to the respective brushes 28.
The inner wall of a square hole 21a of the brush box 21 is formed as shown in FIG. 7 and FIG. 8 which are sectional views taken on lines VII--VII and VIII--VIII, respectively. Stealth grooves 21b in the form of a split circle are longitudinally provided over the whole length of the inner wall of the square hole toward the thickness of the brush 28 so as not to restrict the compression spring 25.
As shown in FIG. 9, the brush 28 inserted in the brush box 21 is disposed so that only contact surfaces 28a as shown by hatching in both side portions of the brush toward the thickness thereof are made to slidably contact the inner walls of the brush box 21.
In the brush holder 20 for the conventional vehicular a.c. generator described above, the stealth grooves 21b are longitudinally made over the whole length of the inner wall of the square hole 21a of brush box 21 toward the thickness of the brush 28. Consequently, only the small contact surfaces 28a in the side portions of the brush 28 are caused to slidably contact the remaining inner wall of the brush box 21. As a result, it becomes a serious problem that the contact surfaces 28a of the brush 28 tend to excessively wear because of the vertical movement of the slip ring 9 caused by vibration, thus resulting in poor contact with the slip ring as the brush shakes and tilts.